Rolled sheet glass is typically formed using a pair of forming rolls. However, conventional glass roll forming machines that employ forming rolls typically produce glass ribbon that does not have a high precision of dimensional uniformity (e.g. thickness uniformity within +/−0.025 mm) and are unable to form thin glass ribbon below 2-3 mm in thickness. One factor that contributes to this lack of precise thickness control is the non-uniform radial thermal expansion of the forming rolls that are being heated by the stream of molten glass that is formed into the glass ribbon.
FIG. 1 illustrates a schematic view of two conventional forming rolls 101, 103 that are not necessarily drawn to scale for clarity. As show, the forming rolls are spaced from each other to define a glass forming gap 105 for receiving a stream of molten glass 107. The forming rolls 101, 103 may expand radially in a non-uniform manner due to heating from the stream of molten glass 107 (which may be about 1000° C. or higher). For example, as indicated by dashed lines 109a, 109b, heating of the forming rolls 101, 103 by the stream of molten glass 107 can cause a working zone surface of each of the forming rolls to expand radially from the corresponding rotation axis in a non-uniform manner across the length of the working zone surface. The non-uniform radial expansion occurs since the central portions of the forming rolls 101, 103 are being raised to a higher temperature than the corresponding end portions of the forming rolls 101, 103. Due to the non-uniform radial expansion, glass ribbon formed by the forming rolls may consequently have a relatively thin central portion when compared to the outer opposite edges of the glass ribbon.
There is a desire to provide forming rolls that may be used to form a glass ribbon with a thickness profile that is not substantially varied due to radial expansion of the forming rolls when heated.